Add site connections endpoints

This adds support for the following REST API endpoints:

- GET `/site/$site/connections`
- GET `/site/$site/connections/$connection_id`
- POST `/sites/$site/connections/$connection_id/delete`

Tests could not be added for deleting connection since endpoints don't exist (yet) for creating a new connection.
@aduth we should create a new connection before to tests, so the we can get it, edit it, and finally remove it.